fix(ci): bound pkgcache _metadata growth to stop macmini disk-fill (#110)

## Problem

The arm64 `build-all` pipeline fills the macmini (gaia) host disk despite an 8h prune.
Root cause is not images or job volumes: it is the persistent dep-cache volume, specifically `pkgcache/R/pkgcache/_metadata`, which grew to ~165 GB.
`{pkgcache}` mints a new content hash for the "patched" binaries repo on every PACKAGES change, so each per-package build writes a fresh ~70 MB `pkgs-<hash>.rds` (+ `patched-<hash>/`) that is never evicted (2407 snapshots observed).
When the disk hits 100% OrbStack stops and the on-host prune can no longer connect to the daemon, so it never self-heals.

## Change (Workstream A of the disk-fill fix)

- Add `trim_pkgcache_metadata()` to `local/r-minor-helpers.R`: keeps the newest `keep` (default 20) `patched-*`/`pkgs-*.rds` entries under `_metadata`, deleting only entries older than `min_age_secs` (default 600s) so it never races the up-to-4 concurrent split-jobs sharing the volume.
Preserves `pkg/` downloads and the stable CRAN/BioC/INLA repo dirs.
No-op when `R_PKG_CACHE_DIR` is empty (amd64) or `_metadata` is absent (first run).
- Call it every 25 packages inside the build loop in `local/build-all.R`.
- Add a defensive start-of-run cleanup of `_metadata/patched-*` + `pkgs-*.rds` to the two workflows that mount the persistent volume (`build-all-versions.yaml`, `build-all-versions-install-deps.yaml`).

Only these paths are touched; `process-updates.yaml`/`weekly-rebuild-missing.yaml` (no persistent volume) are unchanged.

Follow-ups (separate workstreams): on-host self-healing prune watcher + OrbStack disk cap (ansible), and Prometheus/Grafana alerting (k8s-talos).
Upstream: bincraft patched-repo hash churn is the true source fix.

New unit tests (6) for the helper; full suite 24/24 green.

Reviewed-on: #110

local/build-all.R

@@ -122,6 +122,10 @@ s3_cache <- readRDS("/mnt/cache/packages/s3_cache.rds")
 sprintf("S3 cache: %s files", length(s3_cache))
 
 n <- nrow(chunk)
+# Every `trim_every` packages, bound the pkgcache _metadata dir so a full-platform
+# run does not accumulate thousands of ~70 MB snapshots and fill the host disk.
+# No-op on amd64 (R_PKG_CACHE_DIR is empty / cache not persisted).
+trim_every <- 25L
 mapply(
   function(pkg, ver, sens, i) {
     cat(sprintf("[%d/%d] %s_%s (r_minor_sensitive=%s)\n", i, n, pkg, ver, sens))
@@ -147,6 +151,12 @@ mapply(
       upload = TRUE,
       store_build_metadata = TRUE
     )
+    if (i %% trim_every == 0L) {
+      removed <- trim_pkgcache_metadata()
+      if (removed > 0L) {
+        cat(sprintf("  [pkgcache trim] removed %d stale _metadata entries\n", removed))
+      }
+    }
   },
   chunk$Package,
   chunk$Version,